Generally, an amplifier operates to amplify the amplitude of an input signal, such as a high frequency signal. For example, a radio communication apparatus includes a low noise amplifier (LNA) for amplifying a high frequency signal received at an antenna. The signal received at the antenna passes through a filter provided on the reception side and is amplified by the LNA.
The input impedance of an amplifier for amplifying a high frequency signal, such as the LNA described above, is typically limited in the specifications of the amplifier. For example, when the output impedance of an element provided in the preceding stage of the amplifier is 50Ω, the input impedance of the amplifier is also desirably matched to 50Ω. As a result, the high frequency signal input to the amplifier may obtain a maximum electric power.
The amplifier includes a supply resistor for supplying a gate bias voltage to an input terminal. Generally, a noise figure (NF), which indicates how much the amplifier is affected by thermal noise of the supply resistor, is desired to be as small as possible.
To meet the requirements for matching an impedance and lowering the NF as described above, the amplifier includes a feedback path between an output terminal and the input terminal in addition to the supply resistor of the gate bias voltage, which has a given resistance value.
Examples of typical amplifiers include an amplifier that stably operates in transmission and reception states, a cascode-coupled high frequency amplifier, and an amplifier that operates as an output amplifier on the transmission side having multiple stages so that elements on the reception side may not be affected by a signal from the transmission side when the transmission and reception states are switched in a radio communication apparatus.
When the amplifier is switched from the transmission state to the reception state, the bias voltage starts to be supplied in the amplifier. After that, when a direct current (DC) voltage corresponding to a node in the amplifier becomes stable, the amplifier starts to perform desired operations for the signal reception.
However, it may take more time than desired for the amplifier including the feedback path between the output terminal and the input terminal to reach a stable state after the bias voltage has started to be supplied.